This invention relates to a method for controlling the properties of the core when making wallboard. More specifically, it relates to controlling core strength by producing voids of controlled sizes.
Gypsum-based building products are commonly used in construction. Wallboard made of gypsum is fire retardant and can be used in the construction of walls of almost any shape. It is used primarily as an interior wall and ceiling product. Gypsum has sound-deadening properties. It is relatively easily patched or replaced if it becomes damaged. There are a variety of decorative finishes that can be applied to the wallboard, including paint and wallpaper. Even with all of these advantages, it is still a relatively inexpensive building material.
One reason for the reasonable cost of wallboard panels is that they are manufactured by a process that is fast and efficient. A slurry used to form the core includes calcium sulfate hemihydrate and water that are blended in a mixer. As the slurry exits the mixer, foam generated from soap and water is added to the slurry before it is continuously deposited on a paper facing sheet moving past a mixer. A second paper cover sheet is applied thereover and the resultant assembly is formed into the shape of a panel. Calcium sulfate hemihydrate reacts with a sufficient amount of the water to convert the hemihydrate into a matrix of interlocking calcium sulfate dihydrate crystals, causing it to set and to become firm. The continuous strip thus formed is conveyed on a belt until the calcined gypsum is set, and the strip is thereafter cut to form boards of desired length, which boards are conveyed through a drying kiln to remove excess moisture. Since each of these steps takes only minutes, small changes in any of the process steps can lead to gross inefficiencies in the manufacturing process.
Installers prefer lightweight boards to reduce fatigue on the job. The foam introduces voids into the gypsum core that reduce the weight, however if the size of the voids is not controlled, problems with the product may develop. Very large bubbles can cause aesthetic problems. Strength is reduced when many small bubbles leave as many tiny voids in the core. Ideally, a distribution of large and small bubbles is desired to produce a board of high strength yet light weight. In addition to affecting the strength and weight of the finished board, mixing foam into the gypsum slurry decreases the fluidity of the slurry.
Dispersants are known for use with gypsum that help fluidize the mixture of water and calcium sulfate hemihydrate to increase the flowability of the slurry. Naphthalene sulfonate dispersants are well known, but have limited efficacy. Polycarboxylate dispersants are commonly used with cements and, to a lesser degree, with gypsum. The addition of one or more dispersants can be used to increase the fluidity when foam is added.
Further, it has been found that the addition of dispersants to a gypsum slurry changes the size distribution of foam bubbles and the voids they leave behind. Some dispersants cause the boards to have an unusual appearance which may be objectionable to the end user. Other dispersants make very tiny bubbles which can decrease the strength.
Dispersants can also retard the set of the gypsum slurry, further complicating high-speed manufacture of gypsum products such as wallboard. If dispersant dose is increased to improve fluidity, set time may increase. If the wallboard is not sufficiently set at the cutting knife, the product will not hold its shape and will be damaged by handling of the board after it is cut. Reduction in the speed of the line may be necessary to allow the board to harden and maintain its shape.
This complex relationship between the dispersant chemistry, foam bubble size and slurry fluidity makes it difficult to produce a gypsum slurry having both the desired bubble size distribution and fluidity without significantly increasing set time. U.S. Pat. No. 6,264,739 assigned to Kao Corporation, discloses the use of a polymeric dispersant for use in wallboard that stabilizes the foam. This reference describes a polyalkylene glycol monoester monomer having 2 to 300 moles of oxyalkylene groups, each having 2 to 3 carbon atoms and an acrylic repeating unit. The only method disclosed for adding the dispersant is by adding the dispersant to the plaster powder together with surfactants. No mention is made of controlling bubble size or bubble size distribution for controlled core structure, only of stabilizing the foam.
U.S. Pat. No. 6,527,850 also describes a gypsum composition utilizing a dispersant formulation that includes an acrylic/polyether comb-branched copolymer. This reference teaches that there may be a second wet portion of the composition that includes the foam, starch, surfactants and glass fiber. In Example 6, the use of a polycarboxylate comb-branched copolymer together with naphthalene sulfonate is revealed. However, it has been found that if polycarboxylate dispersants and naphthalene sulfonate dispersants are mixed together for addition to a slurry mixer, the components may form a gel that is then very difficult to mix homogeneously into the slurry. The combination of dispersants also resulted in a slurry of lower fluidity.
Thus there is a need for a method to effectively utilize foam and dispersants together to obtain a gypsum core that is both strong and lightweight. Further, the method should accomplish this while maintaining the high fluidity and the set time necessary to efficiently make products.